Trends and hassles in the microbial production of lactic acid from lignocellulosic biomass

Highlights

• This study explores the potential of using lignocellulosic biomass (LCB) in lactic acid (LA) production.

• The efficiency of lactic acid fermentation depends on LCB pretreatment and enzymatic hydrolysis.

• Novel green methods for pre-treatment of LCB for LA fermentation have been highlighted.

• Trends and hassles in LA fermentation using LCB have been discussed.

• Purification techniques and applications of LA in diverse fields have been also reviewed.

Abstract

Lactic acid is an important biomolecule extensively used in various industries for the production of foods, drugs, cosmetics, bioplastic polylactic acid (PLA), etc. Microbial fermentation has proven to be an auspicious approach for lactic acid industrial production because it allows for the utilization of renewable green energy sources. Different biotechnological techniques have been explored to increase lactic acid production. However, this bio-based production route suffers from the major shortcoming derived from the limited availability of fermentable starchy substrates. This concern has led to the search for cost-effective and widely available alternative substrates. Lignocellulosic biomass, due to its abundance and continuous production, is presented as a viable substitute substrate for lactic acid production. However, various challenges stemming from the complexity of polymeric sugars are linked with the use of lignocellulosic materials. The potential solutions to maximize the benefits of using lignocellulosic biomass in LA production should be considered. This review examined and discussed recent progress in the production of lactic acid using lignocellulosic biomass and highlights potential strategies for improving its production. Additionally, the most recent purification techniques are summarized. Building upon this knowledge will set the stage for the further establishment of innovative sustainability approaches to circular bio-economy.

Introduction

Lactic acid (LA) is a robust organic acid with a wide variety of uses in the industrial processing of cosmetics, food, pharmaceuticals, and other commodity chemicals. More than half of the LA produced globally is channeled to the food industry where it serves a broad range of functions as an acidulant, preservative, flavoring agent, emulsifier, and pH regulator in many food products ranging from confectioneries (e.g. sweet, chocolate), dairy foods (e.g. yogurt, cheese), beverages (e.g. beer, wine, soft drinks), bakery products and pickled foods (Eş et al., 2018). Due to its moisturizing, brightening, exfoliating, and anti-aging effect on the skin, LA is present as an indispensable constituent in many cosmetic products (Djukić-Vuković et al., 2019). In the chemical industry, it is applied as a neutralizer, a descaling agent, and due to its antimicrobial potency; it is a major component in personal hygiene products such as sanitizers, cleaning agents, etc., and a monomer feedstock for the generation of other useful chemicals (e.g. acetaldehyde, acrylic acid, propylene oxide, 2,3-pentanedione, propanoic acid, ethyl lactate) and bio-solvents (Abd Alsaheb et al., 2015). Nowadays, lactate esters from alcohols with low molecular weight are used in the formulation of pesticides due to their low toxicity (Li et al., 2020). Additionally, LA can offer the feasibility to generate eco-friendly and biocompatible polylactic acid (PLA) polymers, which recently has increased in its global demand due to its increasing application in the biomedical industry for tissue engineering, medical implants, orthopedic devices, and drug delivery systems (Singhvi et al., 2019). PLA, a biodegradable polymer of LA has also found applications in the production of decomposable food packaging materials, mulch films (for vegetable and fruit crops), trash bags, rigid containers, and other bio-plastics (Krishna et al., 2018). The increasingly diverse use of LA in medicine and pharmaceuticals especially in the formulation of topical ointments, anti-caries agents, chiral intermediates, CAPD (continuous ambulatory peritoneal dialysis) solution, and intravenous (IV) fluids for health management and other industrial sectors (Fig. 1) has led to a surge in overall universal demand for LA (de Oliveira et al., 2018).

At an estimated annual growth rate of 18.7%, the overall demand for LA is envisioned to reach 1960.1 kilotons by 2025 representing about USD 9.8 billion in the international market (Global Lactic Acid Market Size & Share Report, 2019). From a general point of view, the LA price varies depending on its applications (i.e. different purity standards for different applications affect price) and the cost of commodity raw materials used for its processing (Biddy et al., 2016). The current average price of LA ranges from $1.30 - $4.0/kg depending on the supplying geographical region (Pharmacompass, 2016). Hence, the search for novel processes that will guarantee a cheaper LA price is very desirous and a major challenge. LA exists in two enantiomeric forms: L ($+$) and D ($-$) lactic acid. Pure isomers are considered to be of more value than racemic mixtures (DL lactic acid) (Ahmad et al., 2020). The production of lactic acid can be achieved either by biological (lactic acid fermentation) or by chemical processes.

Lactonitrile hydrolysis with strong acids, a process that results in the production of a racemic mixture of D ($-$)- and L ($+$)-lactic acid, is the most common chemical method for producing lactic acid. However, the major limitation of chemical synthesis is that its final product (DL-lactic acid) cannot be utilized in specific industrial applications where only one of the lactic acid isomers is desired. Enantioseparation of racemic mixtures is needed to obtain optically pure LA, which increases production cost. It is also difficult to monitor the physicochemical properties of the end product (Abdel-Rahman et al., 2013). Other limiting factors of chemical production include expensive raw materials and the generation of secondary wastes, which contribute to environmental pollution.

The quest for pure LA (l- or d-isomer), which is required for specific uses in different industries has promoted the microbial fermentation route over the chemical route. Fermentation methods offer many additional advantages as they require renewable resources and mild production conditions as well as involving eco-friendly, cost-effective, and reduced energy processes compared with the fossil fuel-based chemical production of LA (Reddy et al., 2015). Thus, it is not surprising the renewed and growing interest in the synthesis of LA via microbial fermentation. Several lactic acid-fermenting microorganisms, including bacteria, fungi, and yeast have been used for LA production of which the choice of strain is very critical, especially in terms of high optical purity and production capability. Lactic acid bacteria (LAB) and certain bioengineered bacteria species, particularly Escherichia coli and Corynebacterium spp, are widely used for the LA fermentation (Abdel-Rahman and Sonomoto, 2016). Although microbial fermentation is the present and future pathway for LA production, its broad applicability is limited by the high cost of raw materials and low yields. Usually, starch-based feedstocks, such as potatoes, corn, rice, wheat, and cassava, are widely used for LA production but their availability is restricted because they also serve as food, thus affecting their cost. This concern has fueled the search for alternative cheap, abundant, renewable substrates that will guarantee year-round availability of LA (Krishna et al., 2018). In this regard, many studies have suggested the substitution of starch-based materials by lignocellulosic biomass (LCB) as a feedstock for LA production since it serves as an alternative approach to waste management. In addition, the cost-effectiveness and worldwide distribution of LCB could be the driving force for large-scale industrial fermentation (Gunjal et al., 2020). Despite its relative abundance, the structural complexity of LCB hinders the accessibility of fermentable sugars. Therefore, LCB requires pretreatment before it is used for LA production. This essential pretreatment step breaks up the tightly bound structure of lignocellulosic biomass and exposes cellulose and hemicellulose to enzyme action. Other factors such as the generation of mixed sugars (pentoses and hexoses), as well as inhibitor compounds from pretreatment, are amongst the bottlenecks for LA production from LCB. To assure the sustainability of this circular bio-economy approach, these challenges must be circumvented.

The present study discusses the production of LA by using LCB, including the major challenges and strategies to counteract them. Thus, it emphasizes novel green methods for pre-treatment of LCB and enzymatic hydrolysis of LCB for the LA production process. In addition, examples of recent studies that harnessed lactic acid-producing bacteria and LCB in the production of LA and strategies to improve yield are presented.